Organic hydrazides are an essential class of molecules with a wide spectrum of important properties. A sense of its importance can be garnered from the observation that the antitubecular drug isoniazid contains this structural motif. Furthermore, organic hydrazides are versatile building blocks for the synthesis of pharmacologically relevant hydrazones and heterocycles as well as advanced materials. Peptide hydrazides have been widely utilized in bioconjugation reactions to form glyco-conjugates and more recently emerged as powerful precursors for peptide ligation. However, their applicability may be limited in two respects: 1) peptide hydrazides remain challenging to synthesize and require post-synthesis purification, which is a laborious, time-consuming process associated with loss of valuable material; and 2) current peptide hydrazide synthesis strategies are not compatible with all amino acid residues nor their typically modified side-chain derivatives. Therefore, an efficient and mild method for peptide hydrazide synthesis would significantly improve these techniques.
Given the importance of hydrazide derivatives in drug and probe discovery efforts, methods for the solid-phase parallel and combinatorial synthesis of protected hydrazides have been subject to many studies. Peptide hydrazides may be directly synthesized from solid-supported ester-linked peptides by hydrazinolysis (Merrifield, R. B., Adv. Enzymol. 1969, 32, 221. Chang, J. K.; Shimizu, M.; Wang, S. S., J. Org. Chem. 1976, 41. 3255. Merrifield, R. B., J. Am. Chem. Soc. 1963, 85, 2149. Perlow, D. S.; Erb, J. M.; Gould, N. P.; Tung, R. D. Freidinger, R. M. Williams, P. D.; Veber, D. F., J. Org. Chem. 1992 57, 4394. Wang, S. S. J. Org. Chem. 1976, 41, 3258). However, this strategy often requires an excess of hydrazine, which complicates post-cleavage purification. Such hydrazinolysis is neither generally compatible with the Boc/Bzl protecting group strategy, as benzyl esters readily react with hydrazine, nor the base-labile amino protecting groups Phth, For, Fmoc, Dde and Nps, and typically associated with several side-reactions causing low yields of desired peptides, such as those containing side-chain-protected cysteine, aspartic or glutamic acid recidues.
To bypass such problems, synthetic strategies that are based on special hydrazine resins have been proposed. Wang et al. (Wang, S. S.; Merrifield, R. B., J. Am. Chem. Soc. 1969, 91, 6488. Wang, S. S., J. Am. Chem. Soc. 1973, 95, 1328. Wang, S. S.; J. Org. Chem. 1975, 40, 1235) have reported methods for synthesizing peptide hydrazides using alkoxycarbonyl hydrazide resins. Release of peptides from the linkers required 50-95% TFA for 2 hours. Other strategies have relied on commercial trityl and 2-Cl-trityl resins followed by solid-phase substitution of chloride with hydrazine to afford a trityl-based hydrazine linker construct (Stavropoulos, G.; Gatos. D.; Magafa, V.; Barlos, K., Lett. Pept. Sci., 1996, 2, 315-318). Post-synthetic release of compounds was achieved by treating the resin-hydrazides with 1% TFA in AcOH-TFE-DCM (1:2:7). These linkers are nevertheless facing several limitations. Firstly, they do not tolerate C-terminal glutamine, asparagine or aspartic acid recidues (Fang, G.-M.; Li, Y.-M.; Shen, F.; Huang, Y.-C.; Li, J.-B., Lin, Y.; Cui, H.-K.; Liu, L. Angew. Chem., Int. Ed. 2011, 50, 7645-7649. Campayo, L.; Jimenez, B.; Manzano, T.; Navarro, P. Synthesis 1985, 197-200) but also other peptide hydrazides remain challenging to synthesize. Secondly, the use of acidic reaction conditions for liberation of material from the solid support may be a synthetic disadvantage, as some compounds and protecting groups, e.g., Boc and Trt are not compatible with acids. Finally, the use of acid-labile linker strategies also limits the range of chemical transformations applicable to the synthesis of hydrazide derivatives.
Thus, there exists in the art a need for other cleavage principles that are attractive for the introduction of chemical diversity including hydrazide functionality.